IGBTs (Insulated Gate Bipolar Transistors) are very versatile power semiconductor devices since they have a very low on-state resistance, often abbreviated as RON. As IGBTs are often used for controlling inductive loads, power modules which include IGBTs as active switches typically include so-called free-wheeling diodes which allow a current flow in reverse direction. The reverse current may be driven by the inductive load during switching.
An IGBT is a bipolar device. The low RON is the result of a high charge carrier concentration, typically holes, which are emitted from the typically p-doped emitter into the drift region during forward conductive mode of the IGBT. The drift region is then flooded with excess charge carriers. When the IGBT is brought into the blocking mode, the excess charge carriers need to be removed from the drift region before the drift region is capable of carrying the blocking voltage.
Modern IGBTs may have integrated free-wheeling diodes so that no external free-wheeling diode is needed. IGBTs with integrated free-wheeling diodes are also referred to as RC-IGBTs (Reverse Current—Insulated Gate Bipolar Transistor) as the IGBT is adapted to carry also a reverse current.
It is desired that the IGBT can also carry a reverse current when a gate voltage is applied to the gate electrodes of the IGBT cells. Activated IGBT cells, i.e. IGBT cells to which the gate voltage is applied, may however affect the bipolar operation in reverse current mode.
There is therefore a desire to maintain or even improve device performance specifications, while allowing robust reverse current characteristic.